1. Field of the Invention
The present invention relates to a trivalent rare earth ion-containing aluminate phosphor used suitably as a phosphor for light emitting devices such as a fluorescent lamp and a method for producing the same. The present invention also relates to a light emitting device using the phosphor.
2. Description of the Prior Art
An aluminate phosphor such as CeMgAl.sub.11 O.sub.19 :Tb.sup.3+, Ce(III)MgAl.sub.11 O.sub.19, Ce(III)MgAl.sub.11 O.sub.19 :Mn.sup.2+, and CeMgAl.sub.11 O.sub.19 :Tb.sup.3+, Mn.sup.2+ and phosphors based on a compound expressed by a chemical formula Y.sub.3 Al.sub.5 O.sub.12 :Tb.sup.3+ or Y.sub.3 Al.sub.5 O.sub.12 :Ce.sup.3+ are conventionally known examples of trivalent rare earth ion-containing aluminate phosphors for a light emitting device such as a fluorescent lamp.
Methods for producing such a trivalent rare earth ion-containing aluminate phosphor are as follows, for example: a method of heating a phosphor raw material to which a flux to accelerate chemical reaction among phosphor raw materials (e.g., a halogenide such as aluminum fluoride or a boride such as boric acid) is added using an electric furnace (e.g., Phosphor Handbook, p.227 and Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 49-77893); a method of heating a phosphor raw material to which no flux is added (e.g., Japanese Laid-Open Patent Publication (Tokkai-Hei) Nos. 9-151372 and 10-88127); a method of heating a phosphor raw material containing granular .alpha.-alumina powder containing substantially no fractured planes (e.g., Japanese Laid-Open Patent Publication (Tokkai-Hei) Nos. 10-53760 and 10-273656); a method of dripping an aqueous solution mixed with raw material compounds, which can be a raw material for a phosphor, in the form of droplets into a solution cooled to the freezing point or below so as to produce a frozen body of a raw material solution for a phosphor, and heating a granular phosphor raw material obtained by drying the frozen body in a vacuum (e.g., Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 9-291275); a method of heating a granular raw material for a phosphor obtained by spray-pyrolysis of a colloidal solution comprising an aluminum compound and a metal nitrate (e.g., Proceedings of the 3rd International Conference on the Science and Technology of Display Phosphors, p. 257 (1997)); a method of reacting rare earth ions and oxalic acid ions in the presence of an organic base, filtering the precipitated rare earth oxalate and washing it with water, allowing the rare earth oxalate to stand in the air where water vapor is not yet saturated or performing vacuum-drying or lyophilization so as to produce a phosphor raw material containing a granular rare earth compound (median grain size: about 1 to 6 .mu.m) having a spherical grain shape and a uniform grain size distribution and heating the phosphor raw material (e.g., Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 10-88127).
According to the method of heating the phosphor raw material containing the granular a -alumina powder, if .alpha.-alumina powder having a uniform grain size and a spherical shape or a pseudo-spherical shape close to a sphere is used, a trivalent rare earth ion-containing aluminate phosphor having a uniform grain size and a spherical shape or a pseudo-spherical shape close to a sphere can be produced. Furthermore, it is known that according to a method of heating a granular phosphor raw material that can be obtained by vacuum-drying of the frozen body of the phosphor raw material solution or spray-pyrolysis of the colloidal solution, if a phosphor raw material having a uniform grain size and a spherical shape is used, a trivalent rare earth ion-containing aluminate phosphor having a uniform grain size and a spherical shape or a pseudo-spherical shape close to a sphere can be produced.
In the method of heating a phosphor raw material to which no a reactive accelerator is added, it is known that the grain shape of the .alpha.-alumina used for the phosphor raw material is substantially the same as that of the synthesized trivalent rare earth ion-containing aluminate phosphor (Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 9-151372). It also is reported that a spherical .alpha.-alumina powder having a more uniform grain size than that of a conventional one is contained as a part of the phosphor raw material, a trivalent rare earth ion-containing aluminate phosphor having a uniform grain size and a spherical shape can be synthesized.
Furthermore, according to the method of heating the phosphor raw material containing an a -alumina powder having no fractured plane, it is known that not only can a trivalent rare earth ion-containing aluminate phosphor having substantially the same grain shape as that of the .alpha.-alumina powder be obtained, but also can the production yield be kept high (e.g., Japanese Laid-Open Patent Publication (Tokkai-Hei) Nos. 10-53760 and 10-273656).
Furthermore, it is disclosed that the method of heating the granular phosphor raw material (granular phosphor raw material obtained by vacuum-drying of the frozen body of the phosphor raw material solution or spray pyrolysis of the colloidal solution) can produce a trivalent rare earth ion-containing aluminate phosphor having substantially the same grain shape as that of the granular phosphor raw material (e.g., Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 9-291275).
The specific surface area of the spherical aluminate phosphor having a uniform grain size described in Japanese Laid-Open Patent Publication (Tokkai-Hei) Nos. 9-151372, 10-53760, 10-273656, and 9-291275 is half of that of a conventional phosphor having a non-uniform grain size (Proceedings of the 270th Phosphor Conference of Phosphor Society, pp.1-8, Feb. 17, 1998). When such an aluminate phosphor having a uniform grain size is used, the heat deterioration resistance characteristics of the phosphor can improve and the total luminous flux of the fluorescent lamp improves. Furthermore, ion bombardment that the phosphors incur during illumination of the fluorescent lamp can be reduced so that the temporal change of the emission color during lamp illumination can be suppressed. These effects have been reported also by the inventors of the present invention (Electrical Engineers Society study group material, Optical Application and Vision study group, LAV-98-10, pp.19-25, Nov. 25, 1998).
Because of these advantages, aluminate phosphors (preferably, spherical phosphors) having a uniform grain size are regarded as being significant, especially as a phosphor for a small and narrow tube type fluorescent lamp.
The inventors of the present invention also confirmed that the spherical aluminate phosphors having a uniform grain size have the effect of improving the transmission luminance of the phosphor layer or improving the reflection luminance of the phosphor layer by being used in combination with a reflection layer (Proceedings of the 4th Int. Display Workshops, Nov. 19-21, 1997, Nagoya, pp.621-624).
Because of these advantages, the aluminate phosphors (preferably, spherical phosphors) having a uniform grain size are regarded as being most promising also as a phosphor capable of improving the emission intensity of a light emitting device (fluorescent lamp, plasma display, CRT (cathode ray tube), FED (field emission display) or the like).
Hereinafter, a conventionally known method for producing a trivalent rare earth ion-containing aluminate phosphor will be described briefly.
The trivalent rare earth ion-containing aluminate phosphor is produced basically by heating a phosphor raw material so as to effect a reaction in the phosphor raw material. The phosphor raw material is prepared by mixing a plurality of compound materials containing phosphor constituent elements with a mixing machine such as a ball mill. Alternatively, the phosphor raw material can be prepared by dripping an aqueous solution mixed with raw material compounds into a solution cooled to the freezing point or below so as to produce a frozen body of a phosphor raw material solution, and then drying the frozen body in a vacuum or performing spray pyrolysis of the colloidal solution. Alternatively, the phosphor raw material can be prepared by dissolving a plurality of nitrates containing phosphor constituent elements in a water, adding ammonium hydroxide thereto so as to produce a precipitate, and evaporating, drying and solidifying the precipitate.
In general, a halogenide such as aluminum fluoride or a boron compound such as boric acid is suitably added as a flux to the phosphor raw material. However, the trivalent rare earth ion-containing aluminate phosphor can be synthesized without adding the flux.
The phosphor raw material is heated. (fired) in the air or a reducing atmosphere (e.g., a mixed gas atmosphere of nitrogen and hydrogen). The firing may be performed several times repeatedly (e.g., Phosphor Handbook, Ohm-sha, pp. 207-240). In some cases, heating in the air may be performed before firing in a reducing atmosphere. In this case, heating in the air is performed generally at a low temperature in the range from 800 to 1500.degree. C. (preliminary firing), and after preliminary firing, firing is performed in a reducing atmosphere at a temperature higher than that of the air in the range from 1200 to 1800.degree. C. (main firing).
As described above, Japanese Laid-Open Patent Publication Nos. 9-151372, 10-53760, 10-273656 and 9-291275 and the Proceedings of the 3rd International Conference on the Science and Technology of Display Phosphors, p. 257 (1997) disclose a method for producing a phosphor that can provide phosphors that achieve a high performance light emitting device (phosphors, preferably spherical phosphors, having a uniform grain size).
However, with respect to a trivalent rare earth ion-containing aluminate phosphor, such a method cannot provide a phosphor that has both high luminescence performance and a desired grain shape derived from the (.alpha.-alumina powder, which is a part of the phosphor raw material, or a granular phosphor raw material containing aluminum.
Hereinafter, this point will be described in detail. As described in the above listed publications, in the case where a trivalent rare earth ion-containing aluminate phosphor is produced by firing a phosphor raw material, as it is or after preliminary firing in the air at a temperature not more than 1500.degree. C., in a reducing atmosphere at a temperature higher than that for the preliminary firing in the range from 1200 to 1800.degree. C. for several hours so as to effect a reaction in the phosphor raw material, the higher the firing temperature in the reducing atmosphere is, the higher the luminance that can be obtained. However, when the firing temperature is raised, the grain shape tends to be irregular and the grain shape of the .alpha.-alumina powder or the like can be maintained. For this reason, it has been difficult to achieve high luminance while maintaining the preferable form of the grain shape of the phosphor.
For a more specific explanation, FIG. 5 shows the relationship between the firing temperature and the luminance of a CeMgAl.sub.11 O.sub.19 : Tb.sup.3+ trivalent rare earth ion-containing aluminate phosphor produced by the method as disclosed in Japanese Laid-Open Patent Publication No. 10-273656. FIG. 5 shows a luminance relative to the luminance of a CeMgAl.sub.11 O.sub.19 : Tb.sup.3+ green phosphor produced by a method using a flux (hereinafter, also referred to as "commercially available phosphor"), which is expressed by 100.
As shown in FIG. 5, it is necessary to raise the firing temperature to 1700.degree. C. or more in order to produce a trivalent rare earth ion-containing aluminate phosphor having a luminance comparable to the phosphor produced using a flux without a flux. However, the grain shape of the CeMgAl.sub.11 O.sub.19 : Tb.sup.3+ green phosphor that has been fired at such a high temperature is irregular, even if spherical .alpha.-alumina powder having a uniform grain size is used as a part of the phosphor raw material.
In other words, even if the CeMgAl.sub.11 O.sub.19 : Tb.sup.3+ trivalent rare earth ion-containing aluminate phosphor is produced using a spherical .alpha.-alumina powder having a uniform grain size as shown in FIG. 6(a) (e.g., product name: Advanced Alumina produced by Sumitomo Chemical Co., Ltd.) as an aluminum supply source and without adding a flux, the produced CeMgAl.sub.11 O.sub.19 : Tb.sup.3 trivalent rare earth ion-containing aluminate phosphor does not reflect the shape of the .alpha.-alumina powder and the shape thereof is irregular (FIG. 6(c)).
As shown in FIG. 6(d), a CeMgAl.sub.11 O.sub.19 : Tb.sup.3+ green phosphor having a shape reflecting the shape of the .alpha.-alumina powder can be obtained at a firing temperature of 1600.degree. C. However, at this temperature, the luminance level is less than 80% of that of the commercially available phosphor, as shown in FIG. 5.
It is reported that such a problem is not caused when a divalent rare earth ion-containing aluminate phosphor such as one expressed by chemical formula BaMgAl.sub.10 O.sub.17 : Eu.sup.2+ is produced (the proceedings of the 270th Phosphor Conference of Phosphor Society, pp. 1 to 8). The above problem is peculiar to the trivalent rare earth ion-containing aluminate phosphor.
Furthermore, the method of heating a phosphor raw material obtained by vacuum drying of the frozen body of the phosphor raw material solution or spray pyrolysis of the colloidal solution cannot avoid the above problem that the grain shape of the CeMgAl.sub.11 O.sub.19 : Tb.sup.3+ green phosphor that has been fired at a high temperature is non-uniform and irregular.